Fastener driving tools, such as nail or staple drivers, are well known. For example, U.S. Pat. No. 4,403,722 to Nikolich and U.S. Pat. No. 5,090,606 to Torii et al. disclose internal combustion gas-powered fastener driving tools that are portable and self-contained.
Of course, one of the requirements for a fastener driving tool is that it must generate a force that is sufficient to drive a fastener, such as a nail or staple, into the work surface. In many, if not most, applications the fastener is being driven into a solid or hard surface, such as timber framing, concrete or the like. The driving force must therefore be substantial, whether it is developed by the combustion of fuel or by compressed air or by other means. Regardless of the means used to provide the force needed to drive a fastener into an object, it is desirable to provide the greatest amount of force from the resource used, i.e., maximize the efficiency of the fastener driving tool.
In internal combustion powered drivers, the driving force is dependent on proper combustion of the fuel within the tool. More particularly, a persistent issue in the development of an efficient gas-powered tool is reliable ignition of the fuel-air mixture for generation of sufficient power for driving nails or performing other high-power requirement tasks. The flammability limits of propane in air are about 2.2% to 9.5% by volume. When combusted, fuel-to-air ratios in the mid to low end of this range ("lean" mixtures) release the most energy, provide the greatest driving force, and use the fuel most efficiently. Lean mixtures, however, are often difficult to ignite. Fuel-to-air ratios in the mid to high range ("rich" mixtures) release relatively less energy, produce less driving force, and use more fuel per cycle. Rich mixtures, however, are typically more easily ignited than lean mixtures.
The hand tools disclosed in the Torii and Nikolich patents, for example, use a system of baffles or a fan within the combustion chamber to enhance mixing of the fuel-air mixture to provide more reliable and efficient ignition, particularly for lean mixtures. Although the tools shown in Torii and Nikolich may function generally satisfactorily, the internal construction of the tools is complicated, which adds to the manufacturing and assembly cost, as well as to the weight of the device, which is important for portability. Also, internal fans within the combustion chamber may suffer from repeated firing of the device and require more frequent maintenance or replacement.
In addition, combustion may be affected by the presence of gasses or uncombusted fuel that remains in the combustion chamber after firing. In such circumstances it is difficult to accurately control the fuel-to-air mixture in the subsequent combustion cycle, which is required for maximizing the efficiency of the tool. As a result, it is desirable to scavenge or remove as fully as possible the uncombusted fuel and residual gasses from each discharge so that combustion in the next cycle can be more accurately controlled.
It is also important that a fastener driving tool experience a minimum amount of down-time. Routine and proper maintenance of fastener driving tools can help minimize the need for repairs and extend the life of the tool. Evaluating the condition of the tool on a regular basis is essential if breakdowns are to be anticipated in advance and equally important in preventing avoidable damage to the tool.
There is also a continued need to improve the safety of fastener driving tools. Safe work methods cannot always be relied upon to prevent injury or death. Therefore, it is desirable that a fastener driving tool be equipped with safety features to prevent accidental discharge and/or detect whether the tool is being mishandled. Further, it is desirable that the tool include a security mechanism to prevent operation when handled by an unauthorized user, such as a child or thief.